(1) Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, in particular relating to an image forming apparatus such as an electrostatic copier, laser printer, facsimile machine or the like to perform image forming using toner based on electrophotography as well as to an image forming method, in which a developing device that uses a dual-component developer containing a toner and a magnetic carrier is used.
(2) Description of the Prior Art
Conventionally, image forming apparatuses based on electrophotography such as copiers, printers, facsimile machines and the like have been known. The image forming apparatus using electrophotography forms an image by forming an electrostatic latent image on the surface of a photoreceptor, e.g., photoreceptor drum, supplying toner to the photoreceptor drum from a developing device to develop the electrostatic latent image, transferring the toner image formed on photoreceptor drum by development to a sheet of paper etc., and fixing the toner image onto the sheet by means of a fixing device.
Recently, in the image forming apparatuses supporting full-color and/or high-quality images, a dual-component developer (which will be referred to hereinbelow simply as “developer”), which presents excellent charge performance stability, is often used.
This developer consists of a toner and a carrier, which are agitated in the developing device and frictionally rubbed with each other to thereby produce appropriately tribo-electrified toner.
The electrified toner in the developing device is supplied to a dual-component developer supporting member, e.g., the surface of a developing roller. The toner thus supplied to the developing roller is moved by electrostatic attraction to the electrostatic latent image formed on the photoreceptor drum. Hereby, a toner image based on the electrostatic latent image is formed on the photoreceptor drum.
Further, recently, image forming apparatuses are demanded to be made compact and operate at high speeds, hence it has become necessary to electrify the developer quickly and sufficiently and also convey the developer quickly and smoothly.
For this purpose, in order to disperse supplied toner promptly into the developer and provide the toner with an appropriate amount of charge, a circulating type developing device has been adopted in some image forming apparatuses. This circulating type developing device includes: a developer conveying passage in which the developer is circulatively conveyed; a screw auger (developer conveying member) for conveying the developer while agitating the developer in the developer conveying passage; a toner supply port for leading toner from a toner container into the developer conveying passage; and a toner concentration detecting sensor for detecting the toner concentration in the developer. In this arrangement, when the toner concentration in the developer is lower than a predetermined level, a toner supply command is given to the toner cartridge so that toner is supplied to the developer conveying passage and the supplied toner is conveyed whilst being agitated with the developer (see Patent Document 1).
Proposed as another conventional technology is an image forming apparatus, which includes an image density measuring means for measuring image density every pixel in an image formed of a plurality of pixels, and a developer consumption estimating means for estimating the amount of developer to be consumed in development of the image based on the measurements, and supplies the developer by means of a developer supplying means within the period of image forming (see Patent Document 2).
The developer supplying means of Patent Document 2 supplies the developer in such a manner that the amount of developer to be consumed for forming one page of image, which is estimated from part of the image, is supplied to the developing device within the period in which the page of image is being formed. As a result, it is possible to control and supply an appropriate amount of toner on the halfway of printing the page, hence suppress the excess and deficiency errors of the amount of toner.
The developer supplying means in Patent Document 2 is adapted to end or suspend developer supply to the developing device before the predetermined time when the developer agitating means stops agitating. This feature makes it possible to avoid the developer agitating means leaving the developer supplied to the developer agitating means, insufficiently agitated and stopping agitation, so that the developer will have been sufficiently agitated right after the start of next development.
Patent Document 1:
Japanese Patent Application Laid-open 2006-106194
Patent Document 2:
International Publication WO2007/091507
In the aforementioned circulating type developing device using a dual-component developer, if toner to be supplied from the toner cartridge to the developing device is used up, the toner concentration in the developer gradually decreases and the carrier phenomena (carrier adherence) to the photoreceptor drum will occur more frequently, it is hence necessary to perform toner empty detection.
Toner empty detection is to determine (detect) the occurrence of a toner empty state when, for example, the toner concentration of the developer in the developing device, detected by the toner supply detecting sensor does not increase even after a toner supply command was given to the toner cartridge.
However, in the technology described in Patent Document 1, in the case where no toner is supplied even after a toner supply command was given to the toner cartridge because of toner empty in the toner cartridge, if the toner concentration detecting sensor is located away from the toner supply port through which toner is supplied, detection of toner empty is delayed because the fall of toner concentration detected by the toner concentration detecting sensor is sluggish. As a result, there occurs the problem that the occurrence of carrier adherence becomes more frequent.
On the other hand, Patent Document 2 discloses the above-described technology for suppressing variation in toner concentration in the developing device to as low as possible, but there is no description about the toner empty detection.